The present invention relates to industrial processing equipment generally used in the textile industry for the continuous processing of elongated sheets of textile material and more particularly to an improved sieve drum constructed having a single cylindrical layer of a close mesh screen which provides more effective heat treatment of the textile material supported thereon.
Sieve drums are well known in the textile industry where they are employed, usually in tandem, to dry elongated webs of woven or knitted fabrics. Such dryer drums which are generally mounted within an insulated housing are supported for rotation about their axes and comprise substantially impervious end plates connected by a perforated cylindrical surface. A fan mounted in or ducted to one of the end plates operates to create a partial vacuum within the interior of the drum causing air to be drawn therefrom, circulated through a separate heater unit, and drawn back as heated air into the drum through the air-permeable web supported on its perforated cylindrical surface. The web is typically passed over opposing sectors of two tandem drums in a serpentine fashion so that both sides of the web may be directly exposed to the heated air. A stationary baffle provided within each sieve drum blocks the holes in the portion of the perforated surface not covered with the web to ensure that the suction draft of heated air is directed through the web.
The perforated cylindrical surface of known sieve drums have commonly been constructed of a relatively thin metal punctuated with squared, sharply edged holes. This relatively rigid drum surface both supports the web to be dried and transmits torque from one end plate to the other. While such perforated metallic cylinders have produced generally satisfactory drying results, the partial vacuum created within the drums, particularly in higher capacity systems, has caused portions of the web to be partly drawn into the holes as the web passes over the perforated surface creating permanent dimples or puckers in certain types of web materials. In addition, a somewhat similar pattern of drying appears in web materials supported upon these perforated cylinders due to the greater amount of heat being transferred to the web material lying directly over the holes. Such a dotted pattern of heating can be particularly adverse for certain resins, dyestuffs and chemicals commonly used in processing of textile webs.
To resolve these problems of marked impressions and uneven heating of the web materials, it became well known to cover the perforated metallic cylinder with a fine wire mesh which was generally wrapped around the cylinder between the end plates and abutted together by a soldered joint extending along the surface. While such a wire mesh cover overtop the perforated cylinder eliminated much of the undesirable web impressions, there continued to be some surface markings of the web caused by unevenness of the soldered joint in the abutment area. Also, while the degree of uneven, patterned heating showed improvement, there became evident a reduction in air permeability in the joint abutment area that adversely affected drying effectiveness.
An inherent limitation of the perforated cylinder type of sieve drum that remains unaffected by any incorporation of a wire mesh cover is particularly evident when the web materials being heat treated have been treated with resins, dyestuffs and other similar chemicals. Over time, some of the resins are inevitably drawn into the sharply edged, circular holes of the perforated cylinder reducing the effective diameter of the holes and often obstructing them completely. As more and more holes are so obstructed, less and less of the web material is subjected to the suction draft of heated air resulting in ineffective and uneven drying. While certain suction drum designs have been proposed absent the perforated cylinder and having a wire mesh jacket internally supported and secured between opposing end plates, the additional supporting structure required within such suction drums has interfered with the structure and effective operation of the blocking baffles which must be set in close proximity to the interior surface of the drum to function effectively. In addition, existing designs of wire mesh drums have had relatively large mesh openings which adversely affect uniformity of suction draft pressure across the surface length of the drum. Such existing mesh drums, characteristically having their surface area more than 50% open, have produced varied pressure differentials depending upon the length of the drums themselves and the size of the suction draft being employed.